Geochemistry of soil overlying Kopili shale in Dima Hasao District, Assam, India: Implications for pedogenesis, nutrient dynamics and toxic element accumulation

This study investigated the pedogenesis, nutrient availability and toxic element contamination in soil developed over the Kopili shale in Garampani area of Dima Hasao District, Assam, India using geochemistry. Examining soil formation over shale, this research addresses environmental risks of toxic elements, the sustainability of agricultural practices, and the development of soil management strategies. Soil samples collected from pits and trenches were analyzed for major oxide, trace and Rare Earth Element (REE) geochemistry and examined using Pearsons’ correlation coefficient, Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA). The soil has a Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW) and Index of Compositional Variation (ICV) values of 87.40 ± 0.37, 97.96 ± 0.07 and 0.86 ± 0.49, respectively, indicating intense chemical weathering. Mineralogy was dominated by quartz and kaolinite, with notable enrichment in TiO₂, Fe₂O₃, Vanadium (V) and Arsenic (As), while CaO, Na₂O, MgO, Sr, Cu, Mo and Ba were depleted. PCA revealed that soil chemistry is governed entirely by geogenic factors, with no evidence of anthropogenic inputs. Acidic leaching, weathering of heavy minerals and precipitation of iron hydroxides contribute to the overall chemical profile of the soil. Pedogenesis occurs under hot and humid conditions, driven by physical, chemical and biogenic processes. The loss of macro- and micronutrients and high Fe₂O₃, which can potentially hinder nutrient uptake in plants, reduces soil fertility, while elevated As (17.5–145.7 ppm) and V (191–391 ppm) levels pose health risks. The study highlights that in soil with low pH, the concentration of macro-nutrients decreases while toxic elements, such as As increase. Poor soil fertility drives shifting jhum cultivation, contributing to forest loss and increasing landslide risks. Additionally, elevated As levels in the soil significantly contribute to groundwater As contamination in Northeast India.